This class of objects is so tremendously varied that it is not possible to have a collection of “basic data” beyond the simple number of known objects. Each star with planets represents an entire solar system, possibly as complex and varied—and possibly even more so—than our own. This is one of the most exciting areas of modern astronomical research.

Space Exploration for Kids

Overview

When I was studying Astronomy, the only planets that we knew for sure existed were the ones in our solar system. In those days, Pluto was still considered a planet, so the total planet count was 9. In the intervening years, that has changed dramatically. Although Pluto has been demoted to being merely a member of the trans-Neptunian objects, its loss has been more than made up for by the confirmed discovery of over thirty-eight hundred planets orbiting other stars.

And what a ride it has been! Although we still have not confirmed the existence of any planet that could qualify as Earth's twin, we have discovered types of planets that few had ever thought could exist. Giants larger than Jupiter. “Superterrans” that are bigger than Earth, but smaller than Neptune. Planets orbiting so close to their parent stars that years on them are only days—or even only hours. Worlds that are largely water. Worlds where it rains molten glass, or even molten rock. And the list goes on.

One thing to be aware of as you read: Don't expect any pictures. These planets are many light years away, and only a few have ever been imaged, and then only as points of light. We probably won't get any real pictures for a long, long time, if ever.

The gas giants can be further broken down into Jovians—or true gas giants—which are the largest planets, Jupiter and Saturn, and the Neptunians—or ice giants—which are the smaller giant planets, Uranus and Neptune. The terrestrial planets can in turn be divided into Terrans, which are planets the size of Venus and Earth, Subterrans, of which our Solar System has one example, Mars, and Mercurians, which are the smallest, and named after their only member, Mercury. These classifications have been adopted to describe extrasolar terrestrial planets.

In addition, planets have been found that lie in between Terrans and Neptunians, a class of planets that does not exist in our Solar System. Appropriately, they have been dubbed Superterrans. This is only a name at the present, since depending upon their makeup—if they happen to have deep, dense atmospheres—they could actually be more like “mini-Neptunians”. At the time of this article, astronomers have managed to determine both the size and mass of a number of these planets, and as you might guess, what they have found seems to indicate that both types exist. At any rate, we will stick with the accepted nomenclature, until and unless it changes.

Planets are further classified as hot, warm and cold, depending upon their distance from their sun. These definitions have to do with the so-called “habitable zone”, which varies from star to star, depending upon its energy output. Planets that orbit in the habitable zone can have liquid water on their surfaces, assuming appropriate atmospheric conditions. These planets are classified as “warm”. Planets that orbit closer to their sun than the habitable zone are classified as “hot”, and those that orbit further are called “cold”. These classifications apply to all size types.

Gas Giants

Jovians

Back in the old days, before the discovery of extrasolar planets, it was generally assumed that giant gaseous planets only existed in the frozen outer reaches of solar systems. This was based on theories of planetary formation, plus the idea that lighter gases such as hydrogen—which is the principal atmospheric component of gas giants—would quickly escape into space if the planet were heated to the temperatures of the inner Solar System.

This all went by the wayside when astronomers began discovering giant planets that actually orbit closer to their stars than Mercury does to our Sun. In fact, to date, the second highest percentage of extrasolar planets discovered belong in the category of hot Jovians, that is, giant planets like Jupiter or Saturn—or even larger—which orbit in close to their suns. Well over 800 have been discovered at the time of this article. They still present a problem to astronomers working on theories of planetary formation.

Planets have been found that are several times the mass of Jupiter. Theoretically, they should not be larger than Jupiter in diameter; once a planet reaches Jovian size, further mass would tend to cause it to compact upon itself and actually shrink and become denser. Nevertheless, astronomers were further surprised to discover “puffy planets”, which are not only more massive than Jupiter, but up to 3 times the diameter! The most popular theories to date to explain this phenomenon is that the atmospheres of these planets are superheated by their proximity to their sun, and have thus expanded.

To date, astronomers have actually been able to determine general temperature distribution on a few hot Jovians, and have determined the color of at least one. This one turned out to be blue—but not the friendly blue of Earth. This planet probably has molten glass in its atmosphere, which “rains” horizontally in high-speed winds. Speaking of such, scientists have determined that a number of these close-orbiting giants experience extremely powerful winds in their upper atmospheres.

Even more interesting than the hot Jovians are the so-called warm Jovians. These are Jupiter-sized planets that orbit in their sun's habitable zone. These planets are interesting because of the possibility that they may have large moons that could conceivably be Earth-like. At this time, instruments are not yet sensitive enough to detect moons of this size around any extrasolar planet (although one possible moon the size of Neptune has been detected orbiting a Jovian planet). But who knows? When and if that day comes, there may be even more exciting discoveries waiting to be made.

And over three hundred cold Jovian planets have been discovered at outer solar system distances. A few have been discovered that orbit their suns considerably further than Neptune orbits our Sun.

Because the number of hot Jovians already discovered is considerably larger than the number of cold Jovians, some scientists theorize that the hot variety must actually be more common. This is actually unsupportable, for the simple reason that current detection techniques heavily favor finding large planets orbiting close to their suns. For example, if a hypothetical alien astronomer using an instrument equivalent to the Kepler space telescope were in the right position and aimed it at our Solar System, in the same amount of time that Kepler was in operation, he could easily detect Mercury, Venus and Earth—but very likely not all three, because of orbital inclination. Mars could be a “possibility”. On the other hand, there is only a 1 in 3 chance that he could have detected even one transit of Jupiter—and 3 transits are required to confirm the existence of a planet. The odds of detecting the remaining gas giants are even lower.

So regarding the question of distribution of Jovian planets, the simple fact is that we do not have anywhere near enough data to draw any conclusions. But the ones already detected have proven plenty intriguing.

Neptunians

The next class of planet is the Neptunian, which is smaller than the classic Jovian gas giant, but considerably larger than Earth. These planets also have thick, dense atmospheres thousands of kilometers deep, dominated by hydrogen and helium, but with a higher percentage of methane, ammonia and other heavier compounds than Jovians. It is still debated as to whether or not they have a liquid or even semi-solid “surface” deep within, where the atmosphere gives way to the icy mantle. Most scientists believe that the transition between atmosphere and mantle is gradual and not sharply defined. Either way, these planets are definitely not Earthlike.

As with Jovians, astronomers had believed for a long time that Neptunians would only be found in the outer parts of any solar system. And also as with Jovians, they were surprised to discover an abundance of hot Neptunians orbiting in close to their stars. At the time of this article, there are almost as many confirmed hot Neptunians as hot Jovians. For planets orbiting in the zone where liquid water could exist on terrestrial planets, confirmed Jovians far outnumber Neptunians. And in the cold, outer realms, the discrepancy is even huger in favor of Jovians.

However, when it comes to Kepler candidates—that is, possible but unconfirmed planets discovered by the Kepler space telescope— there are nearly fifty percent more possible hot Neptunians as there are hot Jovians. Candidates in the habitable zone and outer, cold zone are more evenly distributed. This difference in numerical distribution may be related to the search methods themselves. The Kepler method favors planets in close to their sun, but detects planets of all sizes equally, except possibly for very small planets. Jovians and Neptunians, both being large, would tend to be equally well detected.

At least one planetary system has been discovered where a Superterran and a Neptunian orbit very close to one another. These two planets almost certainly influence one another gravitationally to a large degree. Assuming that the Superterran possesses a solid surface, or at least a solid mantle and/or crust, it probably experiences strong quakes and volcanism when passing the Neptunian in its orbit.

Terrestrial Planets

Superterrans

This is a class of planet for which we have no examples in our Solar System. Plainly put, these are planets that are larger than Earth, but smaller than Neptune. In general, planets between 2 Earth masses and 10 Earth masses are considered to fall into this category. Among confirmed extrasolar planets, they rank second in number discovered. However, among Kepler candidates, they are the most common type.

The nature of these planets is indeterminate. Some astronomers view them as scaled up versions of Earth, with solid surfaces and proportionately thicker atmospheres. On the other hand, recent measurements using a fairly new detection method have revealed densities for some of these planets that indicate atmospheres thousands of kilometers deep, with probable liquid water surfaces. This would make them more like “mini-Neptunes” than oversized Earths. At this time, astronomers have not yet determined the composition of these thick atmospheres, whether they are composed primarily of hydrogen and helium, or of heavier gases such as nitrogen, carbon dioxide and water vapor. Hopefully, advances in technology will make it possible to find out what these planets are made of. Given the variety of stellar environments that these planets are found in, it may be that both super-Earths and mini-Neptunes will be found, or worlds that are a mixture or even something else completely.

At any rate, many of the confirmed extrasolar planets currently considered to be optimistic candidates for being habitable are Superterrans. Again, because of the uncertain nature of these planets, these candidates must be viewed with a certain degree of skepticism. Speculation is interesting, but we will only have real answers when actual observation becomes possible.

Terrans

These are planets comparable in size to Earth. While size is important in determining whether a planet is Earthlike, it is by no means the only factor. Distance from its sun, atmospheric composition and density, rotational rate, presence or absence of a global magnetic field, and many other factors enter into the equation. In our Solar System there are two Terran-class planets, Earth and Venus. While Venus is similar in size, mass and density to Earth, that is where the similarities end. Earth has an environment friendly and suitable to life. Venus, on the other hand, is the next best thing to Hell itself. See the pages on Earth and Venus for more details.

To date, well over six hundred Terran extrasolar planets have been confirmed to exist, the great majority of them in the hot zone. But more than thirty warm Terrans have been found! In fact, the nearest extrasolar planet to our Sun is a warm Terran. It circles Proxima Centauri, the closest star to our solar system. Unfortunately, Proxima Centauri is known to be an active flare star, and many scientists doubt that this planet could have retained an atmosphere.

Among the most interesting extrasolar planets are the seven that have been confirmed orbiting the star TRAPPIST-1. All seven are either Terran or Subterran (see next section). And three of them lie in the habitable zone. Recent observations suggest that at least one of them may be a water world, with a deep ocean that completely surrounds the planet. At least one other may have a thick water-vapor atmosphere. The two inner planets appear to be Venus-like. But one bit of good news is that TRAPPIST-1, although being a red dwarf, is a relatively quiet star, without much flare activity.

And more Terran planets continue to be found. Among the Kepler candidates, there are over eleven hundred. The vast majority lie in the hot zone, but there are a few in the warm zone. Further observation will be needed to confirm their existence. And beyond that, the recently-launched TESS telescope is just beginning its mission. It is expected to discover thousands more planets—including even more Terrans.

On a final note, remember that even if a planet is the same size and mass as Earth and lies in the habitable zone, there is still no guarantee that it will actually be habitable. If the star is too active, flare activity could strip away the atmosphere, leaving a barren rock incapable of supporting life. And chemical composition is also an important factor. For example, recent studies seem to indicate that solar systems with a much higher fraction of carbon than our own would probably have little or no water available in their inner regions. If this is correct, then you could have a planet that would otherwise be Earth's twin—same size, mass, distance from its sun, etc.—but would be an arid, waterless desert. And still other chemical compositions might produce planetary surfaces and environments that are exotic beyond our imagination, with atmospheres and oceans that we never even dreamed of. Am I dreaming? Perhaps. Only if we ever get out there will we ever know…

Subterrans

Subterran planets are Mars-sized planets. According to scientists searching for extrasolar planets, Subterrans are classified as being potentially habitable. This, of course, would require that the planet have a denser atmosphere than Mars itself, as well as a global magnetic field to shield the planet from radiation and solar flares. According to theory—and supported by results from the exploration rovers—Mars is believed to have had a denser atmosphere in the past. Whether or not it ever had a magnetic field sufficient to shield the planet from flares is still debatable.

Does this imply that it is unlikely that an extrasolar Subterran planet could be Earthlike? Not necessarily. If there is one thing scientists have learned from the study of these planets, it is that there is a far greater variety than anyone had ever imagined. So we will just have to wait and see.

Being small, Subterran planets are more difficult to detect. To date, only 56 have been confirmed, and all but two fall within the hot zone. There are over 260 candidate Subterrans, though. All but one fall within the hot zone. This is almost certainly an artifact of the detection methods; from our own solar system, we know that Subterran planets can exist further from their star. (Mars is considered to lie in the outer part of our Sun's habitable zone.)

As mentioned in the previous section, at least one of the planets in the TRAPPIST-1 system is a Subterran. This one is known as TRAPPIST-1d, and is about three-quarters the diameter of Earth. It is in the zone where liquid water can exist on its surface. However, it may possess up to 250 times the amount of water that Earth has, which would make it a water world, and thus quite different from Earth.

Another possibility is Subterran moons. According to theory, it is highly unlikely that any moon would ever be even as large as Earth (although the possible discovery of a Neptune-sized moon may toss that theory into the garbage can). A Subterran moon orbiting within the magnetic field of a parent Jovian or Neptunian would not need its own magnetic field to shield it, and thus the question of Earthlikeness would be reduced to what type of atmosphere it had, and whether or not it had a rotational period too extreme.

As for life? Well, the same evaluation of the possibility that applied above to Superterrans would also apply here, as it would to any planet with suitable surface conditions. Liquid water may be necessary for life—at least, life as we know it—but its presence does not guarantee that there is life.

Mercurians

These are the smallest of the planetary types. The term “Mercurian” comes from the Solar System example, the planet Mercury, and immediately conjures up images of airless, lunar-like rocks, gray and covered with craters. And given that the only 5 confirmed so far are all in their stars' hot zones, this is almost certainly the case with them. Being the smallest type of planet, they are the most difficult to detect, and at this time there are only an additional 9 candidate Mercurian planets in the list of potential extrasolar planets—again, all in the hot zone. However, there is more to consider.

In our Solar System, Jovians and Neptunians are only found in the outer parts, in the cold zone. However, as has already been discussed, in many extrasolar systems, both types are also found not only in the habitable zone, but in the hot zone as well. So it is equally possible that Mercurians could be found in the habitable zones or cold zones of other stars. In that case—especially in the case of the cold zone—these planets could be totally different animals.

Saturn's moon Titan is larger than Mercury, and if it were not orbiting Saturn, would be considered a planet in its own right. And because of its size, it would be classified as a Mercurian. (Perhaps astronomers would create yet another class of planets, the “Titanian”, but since the current classes are based on size, let us assume that both an independent Titan and Mercury would fall in the same category.) This “Mercurian” would have a different structure, with a considerable portion of its mass made up of water ice, as well as a dense atmosphere. Yet it would be roughly the same size.

Could there be extrasolar “Titans”, independently orbiting their stars as Mercurian-class planets?

At this time, it is impossible to know. Due to their small size and mass, they are difficult to detect when close to their stars, and completely undetectable further out. We will have to wait for the development of new observational technology before we could ever hope to answer this question.

Mercurian-sized moons, on the other hand, should be plentiful. We have 7 such moons in our own solar system. Again, though, the technology does not yet exist for detecting extrasolar moons.

There is one thing that we can be fairly sure of, though. (Note that I said “fairly”; as I said, the Universe has a way of surprising us.) No matter how close or far Mercurians are found from their parent star, they would almost certainly never be Earthlike, due to their low gravity and weak or nonexistent magnetic field.

Stellar Considerations

Another thing to take into consideration when evaluating the suitability of extrasolar planets is the type of star they orbit. Stars range from small red dwarfs a tenth the size of our sun—with a correspondingly small habitable zone—all the way up to blue supergiants. Red dwarfs are by far the most common type of star in the Universe. And many of the planets considered as candidates for habitability by astronomers orbit red dwarfs.

However, red dwarf stars bring a host of problems with them. For one, their habitable zones are very close to the star itself. So close, in fact, that a planet orbiting in the habitable zone would very likely be phase-locked, like the vast majority of moons in our Solar System, thus keeping the same side facing their sun at all times. Theories indicate that this can result in a number of undesirable effects, such as “cold trapping” all the water on the night side, leaving little or none available in liquid form for the rest of the planet. The day side could end up too hot, and the night side too cold, although there would be a ring like the iris of an eyeball where temperatures would be suitable. (Such potentially habitable worlds are referred to as “eyeball Earths”.)

In addition, phase-locking would result in a slow rotation, which would probably preclude the existence of a magnetic field to protect the planet against flares and radiation. And given that red dwarf stars tend to be more active and emit considerably more flares than larger stars such as our Sun, this would almost certainly mean that the planet would be bathed in enough radiation to exterminate any life that might find itself there.

On the other extreme, the habitable zones around blue stars larger than our Sun are correspondingly larger and further out. Planets would not be phase-locked and could rotate like Earth and have appropriate atmospheres and global magnetic fields. As for the presence of life… If life requires a long time to develop spontaneously, then these large stars generally do not live long enough for life to arise before exploding into supernovas or ballooning into red giants. If life arises from intelligent design, then that is another case altogether, although one might ask why a Designer would put life on a planet that will end up being vaporized by its sun in a considerably shorter time than Earth will be.

There are other factors as well. Many stars are variable, with their energy output going up and down either regularly or irregularly. (Our Sun is very slightly variable, but not enough to seriously affect life on Earth.) The majority of stars are not single, but come in pairs or larger groups. The presence of a second sun would add a degree of variability to the energy received by a planet that could definitely have a major effect, just like with a single variable star.

Rogue Planets

Even more interesting are the so-called “rogue planets”, or planets not attached to any star. At one time they were merely objects found in science fiction, but in recent years have become reality. At the time of this article, there are no less than 19 known or possible rogue planets that have been detected. All but one of them are estimated to have at least the mass of Jupiter or more—and the one that does not is only around twice the mass of Earth. Scientists have no way of knowing if these planets were born in interstellar space, or if they were at one time associated with a star, but torn loose by some kind of interaction and ejected from their solar systems.

One would think that these planets would be frozen, out in the interstellar void. However, remember that planets such as Jupiter and Saturn are actually quite hot internally, through heat generated by gravitational contraction. It is very likely that large rogue planets would be similarly hot internally. A really massive rogue planet could possibly even radiate in the infrared, though whether it would be warm enough to heat any attendant moons is unknown.

Recent calculations suggest that even Earth-sized rogue planets would probably retain a considerably thick atmosphere, including hydrogen and helium, since they would not have an external heat source (a sun) to warm them and strip the lighter elements away. These same calculations suggest that even Earth-sized rogue planets may be able to retain a fair amount of heat over long periods of time. This brings up the tantalizing possibilities of dark but warm Earth-sized worlds drifting between the stars, with oceans of liquid water… On the other hand, the density of atmosphere required would almost certainly be detrimental to any living organism on the surface.

Detection Techniques and Observations

As has been mentioned, extrasolar planets are extremely distant, and basically cannot be observed directly. (There are a very few exceptions to this rule, as will be mentioned below.) So detection methods are indirect. Instead of looking to see the planet itself, scientists look at the star to see effects on the star caused by the presence of any attendant worlds. The two main methods are the radial velocity method and the transit method. In addition, there are several other methods that have been used to find the occasional planet.

Radial Velocity Method

When any object orbits another object, neither of the two objects is absolutely fixed in space. In fact, they orbit each other around a common center of gravity. Of course, when one object is orders of magnitude more massive than the other, the center of gravity is usually deep inside the more massive object, near its own center. Nevertheless, if the orbiting object is massive enough, the motion of the main object around the center of gravity can become measurable. This is the heart of the radial velocity method.

When a star moves around a center of gravity caused by an orbiting planet, the star will appear to move back and forth with respect to an observer on Earth. This radial motion causes a slight shift in the spectrum of the star due to the Doppler effect. With very high precision instruments, this motion can be detected.

This method has been the most productive technique to date, in terms of planets detected. It works best with low mass stars, because the difference between their mass and the mass of any planet(s) orbiting them is smaller, and their resulting motions larger. It can detect Earth-sized planets out to around 160 light years, and gas giants several times further. The most common planet type detected by this method is the hot Jovian, because these close in and fast moving planets produce the greatest motions in their parent stars.

There are limitations, of course. Unless the planet also transits in front of its star (see the following section on the transit method), the angle of the plane of the ecliptic (the plane in which the planets orbit) for the target star is uncertain. This means that the calculated mass (the radial velocity method gives masses, but not diameters of planets) will vary according to the sine of the actual inclination.

It is possible to detect multiple planets orbiting stars using this method. The multiple radial signals may appear random and erratic, but the method of Fourier analysis can extract frequencies from the apparent noise, which in turn correspond to possible planets at different distances from their sun. One star, Gliese 581, is known to have at least 4 planets, and possibly up to 6, all detected through the radial velocity method. One of them—unfortunately, one of the unconfirmed ones—is Gliese 581g, which is estimated to be less than 50% larger than Earth. Some have considered it a candidate for habitability, although it is a Superterran, and there are doubts as to the suitability of these worlds.

Transit Method

The Kepler Space Telescope(Artist's Impression)

If a planet orbits in a plane where it passes in front of its sun as seen from Earth, then this transit of the star can be detected as a very slight dip in brightness. This is the essence of the transit method. Given a knowledge of the size of the star, this method can give a fairly accurate measurement of the diameter of an extrasolar planet. The limitations are obvious; if the planet's orbit is inclined even slightly, so that it does not pass in front of its star, then it will not be detected. While many planets have been detected using this method, it is likely that many more have been missed, even in systems where other planets have been found. We can say this with almost total certainty, given that the planets in our own Solar System are slightly inclined with respect to one another's orbits.

In addition to ground-based observations, space telescopes have also been launched to search for planets using the transit method. One was called CoRoT and was sent up by the European Space Agency and managed by the French Space Agency. Another was called Kepler and was launched by NASA. Both returned tremendous amounts of data. Unfortunately, both spacecraft failed in 2013. CoRoT failed completely due to issues with its on-board computer, while Kepler continued to function, but was unable to perform its primary mission because of the failure of two of its four gyroscope wheels. The Kepler mission ended completely in 2018 when the spacecraft ran out of fuel for steering.

At the time of this article, there are more than 4300 candidate planets from the Kepler mission still awaiting confirmation by other methods.

In 2018, NASA launched the TESS (Transiting Exoplanet Survey Satellite) satellite. Similar to Kepler in its function, it focuses on stars closer to Earth than Kepler did. With improved technology, scientists expect that it will find plenty.

The best of both worlds is when a planet not only orbits in a plane where transits can be observed, giving its diameter, but when it also produces a measureable velocity effect on its parent star that can be measured with the radial velocity method. This gives both the size and mass of a planet, and thus allows astronomers to calculate its density, and thus have a good idea as to what the planet is composed of.

Other Methods

There are several other methods, such as orbital light variation, which detects the changes in light reflected from orbiting planets in a manner similar to the phases of the moon, or timing variations, where some variable phenomenon of a given star is affected by orbiting planets and can be measured. Planets have been found around pulsars and certain variable stars by this method. Also, very large, close planets can slightly distort a star, and these distortions affect the amount of light emitted by the star and can be detected.

The gravity of an object can act as a kind of “lens”, affecting the light from stars and galaxies behind it. If two stars—the closer one having planets—are lined up just right, this effect can be used to detect the planets. In addition, this method has been used to detect rogue planets (see section above). One disadvantage is that these measurements usually cannot be repeated, because the line-up only occurs once and has to be precise. So planets “detected” by this method are really only planet candidates, and must await further confirmation by other methods.

As was mentioned above, a new method has allowed astronomers to determine the mass of certain transiting planets. This method is called transit timing variation, and is based on the concept that gravitational interactions between multiple planets will cause the timing of transits across the parent star to vary slightly. This method also holds the possibility of eventually detecting moons around extrasolar planets.

In certain cases, if the planet (or planets) is very large, it has actually become possible in recent years to get direct images. By “direct image”, I am not talking about pictures of the disk of the planet. Rather, these “images” are merely dots of light. But as planets orbit their star, these dots can be observed to move. And when a planet can be directly imaged, it is possible to measure the spectrum from it, and thus determine some of the components of its atmosphere.

Future Missions

At the time of this article, several future space telescope missions are in the works. The first is called Cheops, and is scheduled to be launched by the European Space Agency in 2019. It is intended to be the successor to the successful Gaia mission, which launched in 2013 with the primary purpose of charting a three-dimensional map of the Milky Way. Cheops, while continuing the work of its predecessor, will primarily focus on planet hunting.

In 2021, NASA plans to launch the James Webb Space Telescope, essentially a successor to the Hubble Space Telescope. Optimized for observations in both visual and infrared light, planet-hunting is just one of the many types of astronomical research this telescope is expected to perform.

Beyond 2021 things are a bit hazier. At least two possible missions are in early planning stages; one by NASA that would use a couple of spy telescopes donated by the U.S. National Reconnaissance Office, and the other a European mission similar in nature to Kepler and its successors. As for any more possible missions, we will just have to wait and see.